Ink jet printers have become very popular due to their quiet and fast operation and their high print quality on plain paper. A variety of ink jet printing methods have been developed.
In one ink jet printing method, termed continuous jet printing, ink is delivered under pressure to nozzles in a print head to produce continuous jets of ink. Each jet is separated by vibration into a stream of droplets which are charged and electrostatically deflected, either to a printing medium or to a collection gutter for subsequent recirculation. U.S. Pat. No. 3,596,275 is illustrative of this method.
In another ink jet printing method, termed electrostatic pull printing, the ink in the printing nozzles is under zero pressure or low positive pressure and is electrostatically pulled into a stream of droplets. The droplets fly between two pairs of deflecting electrodes that are arranged to control the droplets' direction of flight and their deposition in desired positions on the printing medium. U.S. Pat. No. 3,060,429 is illustrative of this method.
A third class of methods, more popular than the foregoing, is known as drop-on-demand printing. In this technique, ink is held in the pen at below atmospheric pressure and is ejected by a drop generator, one drop at a time, on demand. Two principal ejection mechanisms are used: thermal bubble and piezoelectric pressure wave. In the thermal bubble systems, a thin film resistor in the drop generator is heated and causes sudden vaporization of a small portion of the ink. The rapidly expanding ink vapor displaces ink from the nozzle causing drop ejection. U.S. Pat. No. 4,490,728 is exemplary of such thermal bubble drop-on-demand systems.
In the piezoelectric pressure wave systems, a piezoelectric element is used to abruptly compress a volume of ink in the drop generator, thereby producing a pressure wave which causes ejection of a drop at the nozzle. U.S. Pat. No. 3,832,579 is exemplary of such piezoelectric pressure wave drop-on-demand systems.
The drop-on-demand techniques require that under quiescent conditions the pressure in the ink reservoir be below ambient so that ink is retained in the pen until it is to be ejected. The amount of this "underpressure" (or "partial vacuum") is critical. If the underpressure is too small, or if the reservoir pressure is positive, ink tends to escape through the drop generators. If the underpressure is too large, air may be sucked in through the drop generators under quiescent conditions. (Air is not normally sucked in through the drop generators because their high capillarity retains the air-ink meniscus against the partial vacuum of the reservoir.)
The underpressure required in drop-on-demand systems can be obtained in a variety of ways. In one system, the underpressure is obtained gravitationally by lowering the ink reservoir so that the surface of the ink is slightly below the level of the nozzles. However, such positioning of the ink reservoir is not always easily achieved and places severe constraints on print head design. Exemplary of this gravitational underpressure technique is U.S. Pat. No. 3,452,361.
Alternative techniques for achieving the required underpressure are shown in U.S. Pat. No. 4,509,062 and in copending application Ser. No. 07/115,013 filed Oct. 28, 1987, both assigned to the present assignee. In the former patent, the underpressure is achieved by using a bladder type ink reservoir which progressively collapses as ink is drawn therefrom. The restorative force of the flexible bladder keeps the pressure of the ink in the reservoir slightly below ambient. In the system disclosed in the latter patent application, the underpressure is achieved by using a capillary reservoir vent tube, or bubble generator, that is immersed in ink in the ink reservoir at one end and coupled to an overflow catchbasin open to atmospheric pressure at the other. As the printhead, which is also connected to the reservoir, draws ink from the reservoir, the internal pressure of the reservoir falls. This underpressure increases as ink is ejected from the reservoir. When the underpressure reaches a threshold value, it draws a small volume of air in through the capillary tube and into the reservoir, thereby preventing the underpressure from exceeding the threshold value.
While the foregoing two approaches for maintaining reservoir underpressure have proven highly satisfactory and unique in many respects, they nonetheless have certain drawbacks. For example, in the pen described in the above-referenced patent, as the flexible bladder reaches its fully collapsed state, the underpressure increases to the point that the drop generator can no longer draw ink therefrom and printing ceases with unused ink left in the bladder. The pen described in the above-referenced application is limited in the temperature and altitude extremes to which it can function properly. For example, if such a pen is transported in an aircraft cabin that is pressurized to an 8000 foot elevation, any air in the ink reservoir will expand in volume by a factor of approximately one third. If the volume of air in the reservoir is more than three times the volume of the catchbasin to which overflow from the capillary reservoir vent tube is routed, the air's expansion will drive more ink into the catchbasin than it can contain and the catchbasin will overflow. This problem can be solved by making the catchbasin large enough to contain the ink in any possible altitude or temperature circumstance, for example, by making the size of the catchbasin fully 35 percent the size of the ink reservoir. However, this solution is volumetrically inefficient and limits the amount of ink that a pen of a given volume can contain.
It is an object of the present invention to provide an ink jet pen that overcomes these problems.
It is a more particular object of the present invention to provide a volumetrically efficient ink jet pen that can undergo arbitrarily large altitude or temperature excursions with an arbitrarily small catchbasin.
According to one embodiment of the present invention, an ink jet pen is constructed with a plurality of ink chambers serially coupled together by small coupling orifices. An ink well extends downwardly from the first chamber and supplies ink to a drop generator positioned at the bottom thereof. A catchbasin extends beneath all of the chambers and is coupled to the last chamber in the series by a drop tube with a bubble generator on the top thereof.
In operation, the plurality of serially coupled chambers that comprises the pen's ink reservoir are initially all filled with ink. As ink is ejected from the first chamber by operation of the pen's drop generator, the partial vacuum induced therein is relieved by ink drawn into the first chamber from the second, which in turn draws ink from the third. The resulting partial vacuum in the third chamber is relieved by the introduction of air bubbles by the bubble generator.
As printing continues, the third reservoir eventually becomes depleted of ink and is filled instead with air introduced from the catchbasin. Thereafter, further printing draws ink from the second chamber into the first and draws bubbles of air from the third chamber into the second. Finally, when the second chamber becomes depleted of ink, further printing simply draws air bubbles into the first chamber from the second.
By the foregoing arrangement, only one chamber contains both air and ink at any given time. The others are filled either with ink or air. Consequently, altitude or pressure changes that cause air in the pen to expand operate on only one of the three chambers to drive ink therefrom, since the others either have no air that can expand or no ink that can be driven. The volume of ink driven to the catchbasin in the illustrated three chamber pen is thus just one third of that in a comparable single chamber pen for any given environmental excursion. Accordingly, the pen of the present invention can be manufactured with a catchbasin only one third the size as required in the prior art, thereby increasing the pen's volumetric efficiency and permitting more of the pen's volume to be used for the initial load of ink.
The principles of the present invention can be applied to pens with an arbitrarily high number of chambers, by which the requisite size of the catchbasin can be reduced to an arbitrarily small volume.
The foregoing and additional objects, features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.